The reaction conditions under which esterification is affected can be varied considerably. The reaction proceeds very slowly at room temperature, but quite rapidly at elevated temperatures. About 99% of the limiting reagent, such as acids, anhydrides or polyols, is converted to an ester within a few hours. Limiting reagents are typically reagents which are not present in stoichiometric excess, e.g., limiting reagents used to make plasticizers include diacids and phthalic anhydride and those used to make polyol esters are polyols.
To facilitate the complete esterification of the reactants, it is desirable that the water which is formed during esterification be removed as rapidly as possible. It is known that water has a detrimental effect upon the rate of conversion. Conventionally, water has been removed by carrying out the reaction in a liquid medium which forms an azeotrope having a boiling point that is lower than that of either component of the reaction. If the resulting ester has a boiling point well above 100.degree. C. at atmospheric pressure, then the reaction temperature can be adjusted such that no liquid medium capable of forming an azeotrope is required.
One conventional process for forming plasticizer esters is disclosed in Great Britain Patent Specification No. 1,426,057 (Imperial Chemical Industries Limited), wherein plasticizer esters are prepared from phthalic anhydride and a C.sub.4 to C.sub.14 alkanol or mixture of such alkanols. For example, a mixture of phthalic anhydride and one or more of these alkanols may be heated gradually up to 180.degree. to 260.degree. C. in the presence of a titanium catalyst (e.g., titanium isopropoxide). When the temperature reaches 180.degree. to 260.degree. C., the esterification is substantially complete although the residual acidity is about 0.3 to 0.05 mg KOH/gram. Aqueous sodium carbonate solution is then slowly added to the ester product to provide 1 to 12 times the stoichiometric amount of alkali. When the temperature has fallen to 150.degree. to 200.degree. C. water or a dilute aqueous alkali solution is admitted and the excess alkanol is removed. By this treatment the titanium catalyst is converted to titanium oxide and precipitated, and thereafter may be filtered off with excess sodium carbonate and the residual acidity is reduced to less than 0.05 mg KOH/gram.
Most esterification processes are capable of converting about 99% of the limiting reagent, such as acids, anhydrides or polyols, to an ester within a few hours; however, after about 90% of the limiting reagent is converted the rate of reaction tends to slow down substantially. It may take half as long again to convert the remaining 4-5% of limiting reagent as it took to convert the initial 95% thereof. Since the chemical industry is continuously seeking to increase the rate of reaction as well as the quality of the resultant esters, it would be quite desirable to develop a process which increases the overall rate of reaction, especially during the esterification of the last 10% of limiting reagent.
In the commercial production of plasticizer esters, e.g., phthalates, adipates, and trimellitates, conversions of greater than 99% and as high as practical are desired. The unreacted portion of the acid or anhydride (i.e., the limiting reagent) will react with base in the final steps of the esterification process and a water soluble salt will be formed that eventually ends up in a waste treatment stream. Thus, an increase from 99.0% to 99.95% conversion reduces waste treatment loads associated with treating unconverted acid or anhydride in plasticizer production by a factor of twenty- In addition, environmental regulations are mandating increases in efficiency of treatment plants. Thus, there is a significant incentive to achieve as high a conversion as possible and reduce the load on existing treatment facilities.
For polyol esters, e.g., esters made from aliphatic acids and trimethyolpropane (i.e., the limiting reagent), the commercially desirable conversions are at greater than 98%. In the case of polyol esters, the excess acid is generally removed by a combination of stripping, neutralization and washing. Thus, the environmental load is determined by the efficiency of stripping prior to neutralization. The conversion level is determined by the product specification for the hydroxyl number, a measure of the number of residual hydroxyl groups in the ester. Typical product applications require conversions of about 98.5% of the original number of hydroxyl groups in the poly alcohol.
It is well known that the amount of water in the esterification reactor at any time is a major factor in determining the overall rate of reaction. The present inventors have developed a novel method by which water in the esterification reactor is removed at a much faster rate than under conventional methods, thereby substantially increasing the overall rate at which the reactants are converted to esters. In general, for a given conversion the lower the water content, the faster the rate of reaction.
The present inventors have modified the conventional esterification process wherein the rate at which water is removed from the reactor is increased to such a degree that the amount of water contained in the reactor after about 90% of the reaction mixture has been converted to an ester is much lower than conventional methods. Modification of other aspects of the process such as neutralization, decolorization or demetalling, water removal, filtration of the ester mixture, steam stripping and filtration of the stripped ester have also been found to enhance the quality of the resultant esters.
The present invention also provides many additional advantages which shall become apparent as described below.